Image display devices that are worn on the head or the face and used to view images, or in other words, head-mounted displays, are known. With a head-mounted display, an image display unit is disposed for each of the left and right eyes, for example, and an enlarged virtual image of a display image is formed by a virtual image optical system, thereby enabling the user to view an immersive image. Head-mounted displays are extremely popular. If mass production advances further in the future, head-mounted displays may become as common as mobile phones, smartphones, or handheld game consoles, and everyone may come to own their own head-mounted display.
When viewing an image on a big screen TV or a head-mounted display, most of one's visual field becomes occupied by that image. Such cases are known to produce motion sickness symptoms, also referred to as virtual reality (VR) sickness. The causes of VR sickness are said to be factors such as a mismatch between the field of view (FOV) of the provided image and the field of view experienced by the viewer, and a mismatch between the depth presented by the image and the depth experienced by the viewer. In particular, VR sickness readily occurs in the case of an image display device used to view images from the first-person view (FPV) or an image display device in which the image is affixed to the viewer's face, like with a head-mounted display.
For example, there has been proposed a VR sickness reduction method including a sickness-coping step that lowers the stimulation strength of a picture of a virtual space presented by a VR system, according to a determination of the user's VR sickness by input of a subjective declaration of VR sickness by the user, or by measurement of physiological indicators.